This needs Squarespace’s Code Injection, which is on the Core, Plus and Advanced plans (and some older Business and Commerce plans). It’s not offered on Basic or Personal.
- Go to Pages → Website Tools → Code Injection. (In the older layout it’s under Settings → Advanced → Code Injection.)
- Paste the Snippet into the Footer box.
- Save.
Your line
<script async src="https://cdn.sitepets.com/v1/pet.js" data-key="sp_YOUR_SITE_KEY"></script>
Once it’s live, Claim your Pet from the browser you copied the Snippet in.